ANSWERS — GOOD QUESTIONS
What is VPD, and why does your greenhouse have to be built for it?
Growers talk about VPD like it is a setting on a controller. It is also a physics problem the whole building either helps you win or quietly loses for you. Here is the buyer's version: what VPD is, the numbers that matter, and why a leaky house makes even great equipment fail.
AMERICAN ROOTS — ANSWERED BY THE PEOPLE WHO BUILD IT · UPDATED JULY 2026
01 — WHAT VPD IS
The drying power of the air, in one number.
Vapor-pressure deficit, measured in kilopascals (kPa), is the gap between how much moisture the air could hold and how much it actually holds — the drying power of the air, and the force that drives water out through the plant (Wikipedia). It folds temperature and humidity into one plant-relevant number, which is why it beats relative humidity: 70% humidity at 18°C and 70% at 30°C are completely different demands on the plant, even though the humidity reading is identical.
02 — LEAF VPD VS AIR VPD
The plant feels the leaf, not the room.
A transpiring leaf runs about 1 to 3°C cooler than the air around it, because evaporation cools it (e-gro bulletin e816). The plant experiences VPD at that leaf surface, so controlling to air VPD alone can leave you off target under strong light. It is a reason precision climate control uses more than a wall thermostat — and a reason the drivers of VPD reach past the equipment into the building itself.
03 — THE BANDS THAT MATTER
A staged ramp: humid for young tissue, drier toward harvest.
1.2–1.6 kPa
THE LATE-FLOWER BAND THAT KEEPS DENSE BUDS TOO DRY FOR ROT — WHILE STILL DRIVING TRANSPIRATION
Industry climate vendors converge on a rising ramp through the crop cycle. Below roughly 0.4 kPa the air is near-saturated and mold moves in; above about 1.6 the plant closes its stomata and growth stalls — so the whole game is holding a moving band between them (climate-control vendors' published bands). A general greenhouse sweet spot sits around 0.45 to 1.25 kPa, often near 0.85 (Wikipedia).
| STAGE | TARGET VPD (kPa) | WHY |
|---|---|---|
| Clones / seedlings | ~0.4–0.8 | Undeveloped roots can't replace water fast; keep transpiration gentle |
| Vegetative | ~0.8–1.2 | Drives vigorous transpiration and nutrient uptake |
| Early flower | ~1.0–1.2 | Begin drying the environment as buds form |
| Mid–late flower | ~1.2–1.6 | Keeps dense buds too dry for botrytis, still transpiring |
OPERATING BANDS, NOT HARD LIMITS — SOURCED FROM CLIMATE-CONTROL VENDORS' PUBLISHED GUIDANCE
04 — WHY IT DRIVES YIELD AND DISEASE
A yield lever and crop insurance at once.
In the band, open stomata let CO2 in and biomass builds. Below it, near-saturated air keeps moisture on the tissue where botrytis and powdery mildew start. Above it, the plant closes up and stalls (Grossiord et al. 2020, New Phytologist; vendor cultivation guidance). The difference between a saleable harvest and a botrytis write-off can be a few hours of the wrong air at the wrong temperature — which is why growers treat VPD as insurance, not just optimization.
05 — WHY THE BUILDING MATTERS — NOT JUST THE BOX
No climate system can hold a setpoint in a house that leaks.
The moisture load is enormous: plants breathe roughly 70% of your irrigation water back into the air, and it peaks at lights-off (HPAC Engineering). A climate system can only hold a VPD band if the envelope seals — which is why our house is sealed by design. Ventilation, even active fan-and-wet-wall ventilation, can only push humidity toward what the outdoor air delivers, so it cannot hold a setpoint on a wet night — the weather sets your VPD for you. The sealed building is what gives the AC system a room it can win in, and that choice is the active-vs-VPD decision — the system itself is engineered in your quote, sized to your room.
PLAINLY
You do not buy VPD control off a shelf. You build a house that lets a system hold it, then size the system to your room. A drafty structure boxes in even the best equipment.
What is a good VPD for cannabis?
It changes by stage — roughly 0.4–0.8 kPa for clones, 0.8–1.2 in veg, and 1.2–1.6 in late flower. The late-flower band keeps dense buds too dry for botrytis while still driving transpiration.
Is VPD better than controlling humidity?
Yes, because relative humidity is meaningless without temperature. VPD folds both into one number that tracks how hard the plant is actually working to move water.
Why does the greenhouse structure affect VPD?
Because no climate system can hold a setpoint in a building that leaks. A sealed, screened envelope gives the equipment a room it can win in; a drafty one lets the outdoor weather set your VPD for you.
Build the room a climate system can win in.
Start with a sealed, screened envelope in the build tool, then size the VPD control to your crop with a Greenhouse Expert.
WIDTH — 30 FT MODULES
OR TALK TO A GREENHOUSE EXPERT — (772) 321-6305